An air bag has been quickly increasing in its installing rate in recent years as one of the safety parts for cars. The air bag is used for such a purpose that, in car crash, a sensor perceives the shock, gas of high temperature and high pressure is generated from an inflator and the air bag is quickly developed by the gas whereby the collision of the bodies or, particularly, the heads of a driver and a passenger to steering wheel, windshield, door glass, etc. is prevented and protected. At present, not only an air bag for a driver seat and a passenger seat coping with the collision from the front side of a car but also a knee air bag which protects the knee, a side air bag or a side curtain air bag which copes with the collision from the lateral side and an air bag which is provided for collision from back side have been adopted. Further, in recent years, an air bag which protects a collided pedestrian has been also known and the using sites of air bags have been steadily increasing now.
While production amounts of air bags are increasing, simplification of an inflator which is combined therewith so as to form an air bag module has been in progress for lowering the cost. As to an inflator, there have been known the so-called stored gas inflator wherein a plug of a metal container in which inert gas such as helium is sealed at high pressure is destroyed by gunpowder so that the gas is released therefrom; the so-called hybrid inflator wherein the gas filled in a relatively small amount is warmed by heat of combustion of gunpowder and the gas generated from gunpowder is combined with the filled gas; and a simple inflator called a pyro-inflator wherein gunpowder which is a gas generator in a solid form is burned. Recently, switching to the pyro-inflator has been in progress.
The pyro-inflator can be made into small size and light weight but, there are a lot of incomplete combustion products generated from gunpowder and floating microparticles due to the residue after combustion of gunpowder. Therefore, temperature of the gas flown into an air bag is higher than that in the conventional inflator and thermal load to a base fabric for air bag is high. Particularly in the so-called impactor evaluation (a method wherein a thing is collided with a developing air bag and the moved distance of the thing is evaluated), the moved distance of a thing is long in the case of an air bag using the conventionally used base fabric for air bag whereby there sometimes happens a phenomenon called “bottoming” (a phenomenon wherein the thing collides with a joining part of a base fabric for air bag).
Up to now, as to an index for succeeding in the impactor evaluation, air permeability of a woven fabric is adopted and it has been common that a base fabric having a low air permeability succeeds in the evaluation (refer to Patent Documents 1 and 2). With regard to air permeability, there have been known the so-called static air permeability in which the air permeability is measured with a constant differential pressure and the so-called dynamic air permeability in which inner pressure and deformed amount of a base fabric changing every moment are measured upon instant application of compressed air to the base fabric. In any of those methods, measurement is carried out at ordinary room temperature. However, in the recent air bag used in combination with a pyro-inflator, it is not always true that abase fabric for air bag having low air permeability at ordinary room temperature (i.e. a base fabric exhibiting high inner pressure as an air bag at ordinary room temperature) succeeds in the impactor evaluation.
It goes without saying that, when an air bag is made of a silicone-coated fabric, it does not fail in the impactor test even if it is used in combination with a pyro-inflator. However, a silicone-coated fabric lacks lightness and compactness and, in view of interior design in a car, such a one is a bit difficult to be used for the areas demanding the compactness such as a driver seat and a passenger seat. Therefore, a non-coated fabric has been still preferably used.
On the other hand, investigations for measures for high temperature inflator gas have been partially carried out as well. As one of them, investigation for a sewn portion of an air bag is also being carried out. However, even in this investigation, no evaluation at high temperature has been conducted and there is a problem that the measure for a pyro-inflator which has been used recently is difficult (refer to Patent Document 3).